This must have become very common, at nearly all the places I went to in July , the guide or tracker asked self guide guests if the police were asking for fines to be paid in cash.
The fundamental advice was don't speed so you could not be pulled over and on the spot speeding fines are not allowed under South African law.
I was told that the officer would issue a ticket from an old book which was kept for private use, the station never saw the book. Any payments made would be kept by the officer. The amount of the fine would be seriously above what is specified by law. Anyone who paid it at the full amount would have a description of their car phoned through to a police friend of the officer at a onward point of their likely journey. tHe next officer would expect full payment at the inflated amount, and call another officer on the likely journey.
The advice was to phone up a lawyer, just invent a number and start talking in a one way conversation about your situation. After a while tell the policeman that the lawyer needed to speak to them , and they would refuse to take the phone.
The only valid way to pay a speeding fine is with a proper ticket at a police station.
The advice was to say that you would pay the fine at a police station . A fair number of people accept the ticket and do not go to the police station. Payment of speeding fines is not covered by international country to country agreements, so you can't be followed up for the unpaid fine at home.
The guides opinion is that this is nothing more than an attempt by some police officers to gather cash for their private use.